Refrigerators can be defined as temperature-cooling machines in which thermal dynamic cyclic processes occur (see, for example, U.S. Pat. No. 2,906,101). A "single-stage"refrigerator includes a single cylindrical work chamber encompassing a displacement member. The chamber is connected in alternation to high-pressure and low-pressure gas sources for predetermined periods, so that the desired thermal dynamic cyclic process (e.g., Stirling process or Gifford/McMahon process) occurs during reciprocation of the displacement member. As a consequence of this reciprocation, heat is withdrawn from a specific region of the work chamber. In two-stage refrigerators (having dual work chambers) employing such cyclic processes, temperatures down to about 10K can be generated using helium as a working gas in the work chamber.
Two-stage refrigerators are often used to operate cryogenic pumps and cryostats. In such devices, the cryogenic pump or cryostat usually includes a housing that receives component parts to be cooled. The refrigerator includes a housing that is connected to the component housing using a connecting pipe. The refrigerator may further include a refrigerating generator having at least one cylindrical work chamber (two in the case of a two-stage refrigerator) with a displacement member oscillating therein. The refrigerating generator extends through the refrigerator housing and the connecting pipe, into the component housing. The portion (usually the cold end) of the refrigerating generator that extends into the component housing carries a plurality of pump surfaces.
In a cryogenic pump operated with a two-stage refrigerator, the first, warmer refrigerator stage carries a pot-shaped pump surface that serves as a radiation shield for the pump surfaces of the second, colder stage. In the work chambers of the first and second refrigerator stages, the displacement members oscillate at a frequency that usually amounts to a few Hertz, for example 2 to 3 Hertz. This oscillation generates vibrations that may be transmitted from the refrigerator through the pump housing and eventually to a recipient connected to the pump housing. In many environments in which cryogenic pumps are commonly used (for example, in electron microscopes), such vibrations are particularly troublesome.
It has therefore already been proposed to provide damping structure that prevents vibrations from the refrigerator from being transmitted through the pump housing to the recipient. German OS No. 36 90 477 and U.S. Pat. No. 4,363,217 disclose damping structure that includes bellows systems combined with various damping agents (for example elastomers, damping material surrounding the bellows, wire suspension systems, and magnetic fields). Damping mechanisms of the type disclosed in these publications are relatively technically complex, and require excessive amounts of space.
European Application No. 19 426 discloses an apparatus in which a cryogenic pump is suspended from an associated recipient in a pendulum arrangement, using a spring bellows. This design, like those previously described, uses an expensive and delicate bellows. Furthermore, such an arrangement is ineffective when the displacement members of the refrigerator reciprocate or oscillate along the axis of distension of the spring bellows.
It can thus be seen that there exists a need for a refrigerator operated apparatus of the type described in which the cryogenic pump housing is coupled to the refrigerator housing with a structure that includes a simple, yet efficient, damping mechanism.